1. Technical Field
The present invention relates to high electron mobility transistors, field-effect transistors, metal-semiconductor (MES) field-effect transistors, metal-insulator-semiconductor (MIS) field-effect transistors, epitaxial substrates, methods of manufacturing epitaxial substrates, and methods of manufacturing Group III nitride transistors.
2. Background Art
Non-Patent Document 1 discloses an AlGaN/GaN high electron mobility transistor having a recess gate structure. The epitaxial layer composed of AlGaN/GaN is formed on the (0001) plane of a sapphire substrate by metal organic chemical vapor deposition. The epitaxial layer includes a GaN nucleation layer having a thickness of 20 nm, an undoped GaN layer having a thickness of 2.5 μm, an Al0.26Ga0.74N barrier layer, an n+-Al0.26Ga0.74N layer having a thickness of 20 nm, and an n+-GaN layer having a thickness of 20 nm. The flow rates of ammonia and triethygallium in order to grow the undoped GaN layer are 5 liters/minute and 69 μmol/minute, respectively. In order to grow the AlGaN layer, the flow rates of ammonia, triethygallium and trimethyl aluminum are 5 liters/minute, 29.5 μmol/minute, and 5.2 μmol/minute, respectively.    Non-Patent Document 1: T. Egawa, et al., Applied Physics Letters, Vol. 76, No. 1, pp. 121-123, Jan. 3, 2000.
Improvement in the characteristics of Group III nitride transistors such as high electron mobility transistors and field-effect transistors has been sought. In order to do so, it is necessary to embody high quality in the channel layer, and to keep the resistivity of the buffer layer high. However, in past research, there has been no concrete method to improve the quality of the channel layer and the resistivity of the buffer layer. According to the research of the inventors, the quality of the channel layer and the resistivity of the buffer layer are related to carbon concentrations in these nitride semiconductor layers. Experiments by the inventors have demonstrated that it is possible to grow the semiconductor by controlling the carbon concentrations in the nitride semiconductor layers. By exploiting this result, transistors having a high-purity channel layer and a high-resistance buffer layer can be manufactured.